1. Field of the Invention
The present invention relates to a pre-emphasis circuit used in a signaling system.
2. Description of the Related Art
Conventionally, research has been conducted on a signaling system based on a differential current driving method, which transmits differential current signals through a differential transmission line pair such that a receiver detects the transmitted signals.
In the differential current driving method, a transmitter generates currents having different magnitudes, corresponding to data which are to be transmitted, and transmits the generated currents through two transmission lines, and a receiver restores the data using a difference between the currents flowing through the two transmission lines.
In the signaling system based on the differential current driving method, when the transmitter transmits a digital signal through a channel with a loss, the transmitted signal is received by the receiver in a state where the signal is distorted according to a frequency characteristic of the channel. In general, since a channel with a loss has a larger loss in a high-frequency region, a high-frequency component of a signal transmitted through the channel is further attenuated than a low-frequency component. In a digital signal, a high-frequency component corresponds to where the voltage level of the signal rapidly changes, that is, a rising edge or falling edge.
Therefore, a signal passing through the channel with a loss has a more distorted waveform than an original waveform, and the arrival time of the transmitted signal may differ for each frequency. As a result, jitter may occur, and a timing margin may be significantly reduced as a whole.
In order to solve such a problem, a pre-emphasis method is used as a method which emphasizes or suppresses a signal for each frequency band to compensate for a signal which is distorted while the signal passes through the channel, and controls the waveform of the signal such that the receiver may receive the original signal.
In the pre-emphasis method, the transmitter previously reinforces a high-frequency band of a signal by a predetermined amount before transmitting the signal, and transmits the reinforced signal. As described above, a high frequency component of a signal emerges at a rising edge or falling edge. The pre-emphasis method is to increase the magnitude of the signal at an edge by an amount distorted through the channel.
FIG. 1 illustrates a conventional pre-emphasis circuit formed in an output driver based on the differential signal driving method.
Referring to FIG. 1, the conventional pre-emphasis circuit includes a first pre-emphasis circuit 110 and a second pre-emphasis circuit 130.
The first pre-emphasis circuit 110 includes four switching transistors 111 to 114 operating in the range between a first voltage VDD and a second voltage GND. Among the four switching transistors 111 to 114, the two switching transistors 111 and 112 are PMOS transistors and operate as a two-stage source to transmit the first voltage VDD to the output driver 120, and the two switching transistors 113 and 114 are NMOS transistors and operate as a two-stage sink to suppress an output voltage of the output driver 120.
The second pre-emphasis circuit 130 includes four switching transistors 131 to 134 operating in a range between the first voltage VDD and the second voltage GND. Among the four switching transistors 131 to 134, the two switching transistors 131 and 132 are PMOS transistors and operate as a two-stage source to transmit the first voltage VDD to the output driver 120, and the two switching transistors 133 and 134 are NMOS transistors and operate as a two-stage sink to suppress an output voltage of the output driver 120.
In order for an effective operation, the pre-emphasis circuit should operate only as a source when operating as a source, and operate only as a sink when operating as a sink.
However, the pre-emphasis circuit illustrated in FIG. 1 controls four transistors using two input signals IN1 and IN2. Therefore, during a source operation or sink operation, an undesired current may be passed to reduce the pre-emphasis effect.
FIGS. 2A and 2B show currents flowing in the switching transistors of the conventional pre-emphasis circuit.
FIG. 2A shows a current flowing in the switching transistor 112 operating as a source, and FIG. 2B shows a current flowing in the switching transistor 113 operating as a sink.
Referring to FIGS. 2A and 2B, it can be seen that the source current and the sink current simultaneously flow in the switching transistor 112 operating as a source and the switching transistor 113 operating as a sink in the conventional pre-emphasis circuit.
Since the conventional pre-emphasis circuit controls four transistors using two input signals IN1 and IN2, the circuit cannot control the source and the sink separately. Therefore, the sink current may flow in the transistors operating as the source, and the source current may flow in the transistors operating as the sink, thereby reducing the pre-emphasis effect.
Furthermore, as the conventional pre-emphasis circuit uses four transistors, a load value is increased by parasitic resistance and parasitic capacitance values depending on the transistors. Thus, the switching speed decreases.
In the conventional pre-emphasis circuit, since a supply time of a pre-emphasis current is decided according to a predetermined delay time, too much pre-emphasis or too little pre-emphasis may be applied according to a load value of a transmission line. Therefore, it is difficult to actively control and supply a pre-emphasis current.